Plugins: Use `include_once` for `wp-admin/includes/plugin-install.php…

…` in `WP_Plugin_Install_List_Table::prepare_items()`.
This brings consistency with the other instances where the file is included, and allows for reusing its functions in custom code.
Props engelen, desrosj.
Fixes #38874.
git-svn-id: https://develop.svn.wordpress.org/trunk@45751 602fd350-edb4-49c9-b593-d223f7449a82